HELP - can't install El Capitan - "item is not available"

Hi

I'm trying to install the GM candidate for El Capitan which i've downloaded off a well known website that starts with im***. I've made a USB boot disk, i works fine, but when i finally get to choose where i want to install EL CAPITAN, it asks for my iCloud login & password, and then says "item is not available - please try again later"

Did i download a wrong version ? Will it work later ? I'm not a dev btw.

any help ?
thanks